High speed rotary cutting in machine tools presents several different problems in the design of machine tool assemblies, and particularly the spindles and the bearings utilized therewith. Herein high speed machining operations refers to machine rotary tool speeds in the range of over 30,000 revolutions per minute and up to 40,000 revolutions per minute. Conventional rolling metal-to-metal contacting bearings or ball bearings utilized for spindles limit the rotary speeds at which the spindle can be driven by their DN value which is the product of the bearing inner diameter (D) in millimeters multiplied by the rotary speed of the tool in number of revolutions per minute (N). Larger spindles are typically utilized for handling the increased loads generated by high speed operations. These spindles have greater mass and create higher unit loads on their bearings which are also larger to accommodate the larger spindles and their increased load bearing requirements. Conventional contacting bearings utilized in machine tools have a usual upper limit on their DN value of 1,000,000 so that at larger spindle bearing diameters they are pushed to their limit with respect to their DN value when run at high speed. At these high speeds, special precautions must be taken as to the bearing materials, lubrication and mounting to minimize the effects of increased friction and heat, resulting in increased expense associated with the bearings. Also, during high speed operations, any imperfections in the contacting surfaces of the conventional bearings, such as caused by wear or sudden loads on the spindle, will be magnified and can cause bearing failure. In this regard, efforts have been made to reduce the size of the spindle to correspondingly reduce the diameter of the spindle bearings to more readily allow the spindle rotary speed to be increased, as in U.S. Pat. No. 5,322,494 to Holtey, et al., commonly assigned to the assignee herein. However, even with the smaller spindle design in the '494 Patent, conventional ball bearings, if used in this design at speeds of 30,000 to 40,000 rpm, will wear and eventually fail due to the extreme conditions generated during high speed operations, including increased friction, heat and centrifugal forces on the balls. As the bearings wear, increased levels of vibrations can occur reducing machining accuracy. Vibrations can also cause dings to develop in the contacting metal which, in turn, will cause the ball bearings to rapidly wear. Bearing wear leading to eventual failure and system crashes raises operating costs by the increased production downtime as well as the expenses associated with the required maintenance for rebuilding the bearings.
Thus, it is desirable that with the high speed machining described above of over 30,000 rpm, the spindle of the machine tool have a high load capacity with increased dynamic stiffness for providing stable and accurate operation at the high speeds where contacting bearings will eventually fail. Hydrodynamic fluid bearings are known for use with spindles to provide the spindle with a non-contacting bearing system where fluid is flowed through the bearing so that a self-generating pressure film is developed in the bearing. These pressures can be sufficient to sustain a considerable load and to keep rubbing surfaces of the bearing separated. In fluid bearings, the gradual wear problem experienced with ball bearings is substantially eliminated as there is no metal-to-metal contact when the tool is operated. In addition, catastrophic bearing failure or "crashes" will only occur in extreme instances such as where an unusually large load for a long duration decreases the fluid film to a minimum or the fluid supply fails. This is in contrast to the situation with mechanical contacting ball bearings which can undergo catastrophic failure due to lubrication failures, excessive loads or material failures or breakages of some sort. In addition, momentary overloads can damage the contacting bearings leading to their failure whereas, with fluid bearings, as the bearing clearance is reduced during the momentary overload, the load carrying capacity of the bearing increases. This is due to the "squeeze-film load effect" with these types of momentary overloads. Where the overload is of a very brief duration, e.g., fractions of a second, the fluid that is trapped in the bearing will substantially stay in place because there is not enough time for the fluid film to be squeezed out of the bearing so that as long as a sufficient fluid film is present, there will be no metal-to-metal contact and no failure of the fluid bearing. In this manner, momentary overloads will not effect the fluid bearing at all as opposed to their damaging consequences when contacting roller-type bearings are used.
It is known to form hydrodynamic bearings with a lobe profile as with a cylindrical bearing, where the fluid film has a constant film thickness all around the spindle journal shaft, instability can result unless the load to be taken by the spindle is only unidirectional. With high speed cutting operations of over 30,000 rpm, the design of the lobe profile is particularly critical and is highly dependent upon the application parameters, including the bearing diameter, loads, spindle shaft speeds and the bearing fluid viscosity. The load carrying capacity for hydrodynamic bearings is in direct proportion to the viscosity of the fluid utilized. Taking these considerations into account, the lobe profile has to be designed appropriately so that the lobe portion of the bearings converging from the spindle is not too large so that there is not enough load carrying capacity in the bearing or so small so that there is too much heat generated in the bearing when operated at high speeds. Lack of sufficient load carrying capacity can cause instability and inaccuracies during machining and reduce the life of the bearings or lead to premature bearing failure. High temperatures can cause expansion of the spindle and bearing leading to machining errors caused by thermal deformation of the machine tool components.
As previously mentioned, it is important that the machine tool system have good load carrying capacity and high stiffness for accurate machining even at the high operating speeds contemplated herein. At the rotational speeds of the spindle shaft of over 30,000 rpm, vibrations can be a significant problem with respect to machining accuracy and bearing wear, as earlier discussed, especially if they get to the point where they are in tune with the shaft bending frequency. In this regard, it is desirable to design the hydrodynamic bearing pads to provide the spindle shaft with a high degree of stiffness so as to avoid the onset of high speed resonant vibration of the spindle shaft.
An additional problem with fluid bearings is that where different bearing and cutting fluids are utilized, proper sealing between the different fluids must be present. Thus, when an oil-based bearing fluid is used, any loss of sealing from the cutting fluid, which is typically a water-based fluid, is to be avoided. However, at high rotary speeds of over 30,000 rpm, conventional contacting seals will quickly wear to the point where their sealing function is lost so that contacting seals cannot practically be used at the high operating speeds to seal and isolate the oil-based bearing fluid from the water-based cutting fluid and prevent potential contamination of the cutting fluid with the bearing fluid. Accordingly, it is desirable to provide for high speed machining operations where the seal utilized to keep the bearing fluid from the cutting fluid will not fail at high operating speeds and, if there is any leakage of the bearing fluid into the cutting fluid, it will not hurt the lubrication and cooling functions of the cutting fluid by mixing a different fluid therewith.
The typical tool holder is too large in terms of its mass and size for use with the high speed spindle design herein. In these prior tool mounting arrangements, the cooperating tapers providing on the tool holder and in the spindle were fairly large such that the bearings either had to be too large reducing the speed with which the spindle could be driven, or resulted in a greater cantilever out forwardly from the front bearings on the spindle, increasing the bearing load and instability of the machine tool. Also, with larger size and mass tool holders, the time required for accelerating them up to the desired cutting speed and decelerating them down upon completion of a cutting operation for tool changeover is increased. Where several successive and different machining operations requiring different cutting tools takes place, any lost time such as that due to increased time for accelerating or decelerating the tool to and from the desired speed leads to machining inefficiencies. Accordingly, there is a need for a tool holding system which can be utilized in the present high speed machine tool assembly which is not so large that it cannot be driven at high rotary speeds of operation or generate instability in the tool. The tool holder needs to be able to be coupled to the spindle for stable high speed rotation therewith in a rigid and stiff manner. In addition, the tool holder should be such that it allows for rapid tool changeover on the spindle.